Coke, which is used in a blast furnace method which is the most common method for manufacturing pig iron, has roles as a reducing agent of iron ore, a heat resource and a spacer. As it is important to keep gas permeability in a blast furnace in order to operate a blast furnace steadily and efficiently, the manufacture of coke having high strength is desired. Coke is manufactured by carbonizing a coal blend, which is made by blending various kinds of coal for coke making which are pulverized and whose particle size has been adjusted, in a coke oven. Coal for coke making softens and melts at a temperature in the range from 300° C. to 550° C. when carbonization is performed, and, at the same time, foams and swells due to the generation of volatile matter, which results in the particles of the coal adhering to each other so that the coal is made into agglomerated semi-coke. Then, the semi-coke is made into strong coke by being made denser in a process in which the semi-coke is heated up to a temperature of about 1000° C. and shrinks. Therefore, the adhesiveness of coal when softening and melting occur has a large influence on the properties of coke such as strength and particle size after carbonization has been performed.
As the properties and the cake structure of coke widely vary depending on the thermal plasticity of coal after carbonization has been performed as described above, the thermal plasticity of coal is so important that investigations on a method observing the thermal plasticity have been actively conducted since long ago. In particular, coke strength, which is an important property of coke, widely varies depending on the properties of coal, which is the raw material of coke, especially on coal rank and thermal plasticity. Thermal plasticity is the quality of softening and melting when coal is heated, and is usually estimated by observing the fluidity, the viscosity, the adhesiveness, the swelling property and so forth of a thermal plastic material.
The examples of common methods for observing fluidity when softening and melting occur, which is a kind of thermal plasticity, include a method for testing fluidity of coal by using a Gieseler plastometer method in accordance with JIS M 8801. A Gieseler plastometer method is a method in which coal is pulverized into a particle size of 425 μm or less, then the pulverized coal is put into a specified retort and heated at a specified heating rate, and then the rotational velocity of a stirring stick on which the specified torque is applied is observed on a scale plate and represented in units of ddpm (dial divisions per minute).
While the rotational velocity is observed for a constant torque in a Gieseler plastometer method, a method in which a torque is observed for a constant rotational velocity has also been developed. For example, Patent Literature 1 discloses a method in which a torque is observed while a rotator is rotating at a constant rotational velocity.
In addition, there is a method for observing viscosity by using a dynamic viscoelasticity observing machine in order to observe viscosity, which has a physical meaning, as thermal plasticity (for example, refer to Patent Literature 2). A viscoelastic behavior is observed when a cyclic force is applied to a viscoelastic body in the observation of dynamic viscoelasticity. Patent Literature 2 discloses a method in which thermal plasticity is estimated in terms of complex viscosity among observed parameters and which is characterized in that the viscosity of a thermal plastic coal can be observed at an arbitrary shear rate.
Moreover, an example, in which adhesiveness of the softening coal to activated carbon or glass beads is observed as thermal plasticity of coal, has been reported. This is a method in which activated carbon or glass beads are put on the upper and lower surfaces of a small amount of a coal sample, then the sample is heated, then cooled after softening and melting has occurred, and then the adhesiveness of the coal sample to activated carbon or glass beads is observed from the appearance.
An example of common methods for observing the swelling property of coal when softening and melting occurs is a dilatometer method in accordance with JIS M 8801. A dilatometer method is a method in which coal is pulverized into a particle size of 250 μm or less, then the pulverized coal is compacted in a specified method, put into a specified retort and heated at a specified heating rate, and then the time change in the displacement of the coal is observed by using a detection stick which is placed on top of the coal.
Moreover, a method for testing the swelling property of coal, in which the permeation behavior of gas which is generated when softening and melting occur is improved in order to simulate the thermal plastic behavior of coal in a coke oven, is well known (for example, refer to Patent Literature 3). This is a method in which the observation environment is made closer to one in which swelling behavior is observed in a coke oven by placing a permeable material between a coal layer and a piston, or between a coal layer and a piston and under the coal layer in order to increase the number of permeation channels for volatile matter and liquid material which are generated from coal. Similarly, a method for observing the swelling property of coal, by placing a material having channels permeating through the body on top of a coal layer and by heating the coal with microwaves while a load is applied to the coal, is well known (refer to Patent Literature 4).